Re: [PATCH 00/27] [RFC] Sparse Index

"Derrick Stolee via GitGitGadget" [off-list ref] writes:

This RFC proposes an update to the index formats to allow "sparse directory
entries". These entries correspond to directories that are completely
excluded from the sparse checkout definition. We can detect that a directory
is excluded when using "cone mode" patterns.

Yay.

Since having directory entries is a radical departure from the existing
index format, a new extension "extensions.sparseIndex" is added. Using a
sparse index should cause incompatible tools to fail because they do not
understand this extension.

Safety is good, but because the index is purely a local matter, we
do not have to be so careful as updating the network protocols or
pack/object formats.

I think the use of "extensions.*" mechanism to render the repository
that uses the new feature unusable by older Git is safe enough, but
it may be too draconian.  For example, when things go wrong, don't
you want to "fetch"/"clone" from it into another repository to first
save the objects and refs?  You do not need a version of the index
file you understand in order to do that.

The index format has a mechanism to make older versions of Git bail
when it encounters a file that uses newer feature that they do not
understand.  Perhaps using it is sufficient instead?